Sarang, Ian pointed out the major reason why subs, for the most part, don't make use of non-parallel sides for the enclosure. The small "room" that the inside of the enclosure forms follows the same room mode rules as do our listening rooms. If the modes that would exist in that "room" are well above the frequencies that the sub actually plays, then reducing or spreading them out wouldn't be of benefit. For example, calculate the modes for a room say 1.2'x1.5'x1.8', or more conveniently, use a calculator such as this one . Note that the resulting standing waves inside the subwoofer "room" are all in the mid-range and far above what frequencies are actually played.

For loudspeakers though you would want to avoid resonance frequencies in the mid-range at the very least. For subwoofers though how can it be a bad thing to raise the resonant frequency?

That said some of the best possible performing loudspeakers like the Revel Salon 2 have very, very inert cabinets, on the opposite spectrum, the Infinity Primus are basically hallow cabinets but still offer very good performance. Controlling cabinet resonance I think would depend on a lot of factors and not one that is a matter of being absolute. Its like a lot of tradeoffs in loudspeaker design.

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